Some of the fundamental existential impacts of the solar cycle were certainly understood by the Neolithic people who built Newgrange and were able to align the gigantic construction with the position of the sun rise at winter solstice. It was a point of return in “the wheel of time”, the annual cycle of “ageing, rebirth, and rejuvenation of Nature”. But how living individuals reproduce and come into being was a mystery right up to modern times. The Roman writer in natural history, Pliny (ca 70 AD), for instance stated that: “…after six months’ duration , frogs melt away into slime, though no one ever sees how it is done; after which they come to life again in the water during the spring, just as they were before. This is affected by some occult operation of Nature, and happens regularly every year. Mussels, also, and scallops are produced in the sand by the spontaneous operations of nature.”
Although the famous experiments by Francesco Redi had refuted some ideas about “spontaneous generation” in the mid 16-hundreds, the concept was still an important part of Lamarck’s theory of evolution that was opposed by his colleague Cuvier. Birth, of course, has also been a subject of discussions when pondering the mysteries of the Mary cult: was it really a case of parthenogenesis? What is really going on in the making of a body – the “process of incarnation”?
Botryllus schlosseri (photo: K. Kongshavn)
Botryllus schlosseri, the “golden star tunicate”, is a common species on Atlantic coasts and recently has expanded its distributions to other seas as a result of human marine travelling. Researchers at the University of Bergen (Delsuc et al 2006) found that the tunicates belong to an evolutionary lineage that is the closest to vertebrates (including humans). B. schlosseri is relatively easy to keep in aquaria and has taught us a lot about reproduction and life cycles.
The similarity between the tunicates and the vertebrates are only apparent in the early stages of tunicate life. The larvae have a body with a tail containing the “chorda”, and a dorsal nerve tube, – both unique characteristic features of the Chordate animals (see figure 1A in in Voskoboynik el al. 2013). But these similarities disappear within a few hours when the free swimming larva has settled on some surface substrate and started the metamorphosis into the sack like body of an adult tunicate with a filter feeding gut. The larva was the result of sexual reproduction, the merged genetic material from sperm and egg. However, the metamorphosed individual will soon begin to reproduce asexually by budding off a copy of itself in a neighbouring position. The results of such multiplications are clusters of two to 12 genetically identical individuals in a star like pattern. These individuals, called zooids, are active for relatively short time, about a week at 19 oC, until they become inactive and gradually are reabsorbed by other cells in the colony while being replaced by new zooids. This sort of programmed cell death is called apoptosis and researches believe that studies of B. schlosseri can reveal some of what is going on with ageing and death of cells. It has been estimated that in an adult human body there is apoptosis of about 50 to 70 billion cells per day. Fortunately there is also renewal of cells, like in the growing colony of Botryllus. Very interesting things may happen if the zooids from different larvae are meeting up at the margins of two colonies with the so-called ampullae. Botryllus has a self-recognition system that is controlled by just one gene, but the gene occurs in many variants (alleles). If the alleles from two colonies are compatible, the blood vessel systems of the two colonies may grow together so that one colony is actually formed by zooids with different genetics. This is somewhat analogous to what happens between mother and child in the mammalian placenta. If the compatibility of two colonies is bad, they will “fight” each other in an inflammatory immune reaction. Such processes have special interest with respect to understanding immune systems and the outcome of organ transplantation.
It takes about 3-4 weeks for a colony to become sexually mature so that egg and sperm may be released in turn, avoiding self-fertilization. The duration of a colony is believed to be about 12 to 18 months in Norwegian waters (Moen & Svendsen 2008).
The reproduction system of B. schlosseri is just one of many different reproduction systems of animals. Where does individuality begin and stop? Would a zooid greet its neighbour with “Merry Christmas, I!”?
Delsuc et al. (2006). Tunicates and not cephalochordates are the closest living relatives of vertebrates. Nature: 439:965-968.
Manni et al (2007). Botryllus schlosseri: A model ascidian for the study of asexual reproduction. Developmental Dynamics 236(2): 335-352.
Moen & Svendsen (2008) Dyreliv i havet. KOM Forlag.
Tiozzo et al. (2006). Programmed cell death in vegetative development: Apoptosis during the colonial life cycle of the ascidian Botryllus schlosseri. Tissue and Cell 38 (3): 193-201
Voskoboynik et al. (2013) The genome sequence of the colonial chordate, Botryllus schlosseri Elife. DOI: 10.7554/eLife.00569.001